Free Space Optical (FSO) communications systems are well known for their ability to provide high data rate communications links. A FSO communication system typically consists of a pair of nodes or communication devices. Each node typically comprises an optical source, for example a laser or light emitting diode (LED), and an optical receiver. In use, the optical source of each node is aligned with the optical receiver of the other node. Modulation of optical signals emitted by the optical sources allows for the bidirectional transfer of data between the two nodes. Thus, there is a datalink between the nodes.
Most current FSO systems are mounted in fixed positions on the Earth and are manually aligned with each other. Commercial systems are available which can offer data rates of several Giga-bits per second (Gbps) over a range of several kilometres.
It is desirable to have communications systems that allow for underwater communications. Radio Frequency (RF) signals tends to be heavily attenuated by seawater, and hence the range of RF communications systems tends to be severely limited. Acoustic systems can offer low data rate transmission (kbps) over long ranges, but are typically overt which is undesirable for certain applications.
Optical communications systems have also been developed for underwater applications. Such underwater optical communications systems tend to provide relatively high data rate communications over short to medium ranges, for example up to a 300 m or so.
Like land-based FSO communication systems, an underwater optical communication system typically comprises a pair of nodes, each node will comprising an optical source and detector. However, unlike land-based FSO communication systems (where node positions are typically fixed), underwater modes tend to be mobile. Hence, in underwater applications, the position of each node, and hence the range and angular separation between the nodes, is not fixed. Thus, in underwater applications, alignment between opposing optical sources and detectors tends to be required in order for the communications system to function. In addition, many underwater nodes are unmanned nodes (e.g. unmanned vehicle), and hence manual alignment between a pair of nodes tends not to be possible. Furthermore, while each underwater node may have some estimate of the relative location of the opposing underwater node (e.g. a pre-programmed location and navigation using GPS/inertial systems, or through use of a separate data link) there may be a large range and angular uncertainty in its position.
Hence, a strategy is required for each node to accurately acquire the location of the opposing node before beam alignment (and hence optical communications) can occur. In addition, since the platforms may still be mobile, active beam alignment may be required during data transfer (tracking).